1. Technical Field
The present invention relates generally to a covering, and more particularly to a lifting device of a cordless covering.
2. Description of Related Art
There are various coverings for windows and doors, some of which are cordless coverings. The cordless covering has no cord for user to pull to lift a covering material. Since no cord is left on the cordless covering, it may avoid the problem of strangling kids, and the covering looks fancier without the cord.
A conventional cordless covering is provided with an automatic lifting device to lift or lower a bottom rail of the covering material. The covering material could be slats, a cellular shade, a pleated blind, or other shades and blinds. No matter what kind of the covering materials is used, the covering is provided with lifting cords extending out of a headrail, passing through the covering material, and then fastened to the bottom rail. The lifting cords are connected to the automatic lifting device in the headrail. The automatic lifting device may reel in or out the lifting cords to lift or lower the bottom rail so as to lift or lower the covering material.
However, it would get heavier while the bottom rail and the covering material are being lifted, but the conventional automatic lifting device only provides a constant lifting force. Therefore, the user has to push the bottom rail harder to assist the automatic lifting device. The other problem is that the bottom rail will never stop at the desired position, usually at a lower than desired position, because the weight of the bottom rail and the covering material will move the bottom rail downwards.
Take a cellular covering for example, the cellular shade has many chambers therein which make the cellular shade compressible. However, the cellular shade generates an internal shrinking force when it is extended. For this reason, some manufacturers provide the bottom rail some extra weights to offset the shrinking force of the cellular shade. But the heavy bottom rail makes the product heavier, the user have to push the bottom rail harder to lift the bottom rail and the cellular shade, and it still has the problem that the bottom rail can not stop at the desired position, but lower than that.